For example, the car body of an automobile includes various structural members (examples: floor cross members, side sills, side members, etc.). Press formed products, which use metal sheets such as steel sheets as their starting materials, are heavily used in the structural members. Due to the advantages over material rupture, shape fixability, and producing cost at the time of press forming, in many cases, a press formed product is obtained by performing press working on a metal sheet through bending forming. The press formed product has, for example, a channel cross-section shape or a hat-shaped cross-section shape. The press formed product having a channel cross-section includes a top panel part and a pair of vertical wall parts extending from the top panel part. The press formed product having a hat-shaped cross-section further includes a pair of flange parts extending from the respective vertical wall parts.
FIG. 1 is a cross-sectional view schematically showing a configuration example of a press forming apparatus that performs general bending forming. As shown in FIG. 1, a press forming apparatus 1 is an apparatus for producing a press formed product having a channel cross-section or a hat-shaped cross-section. FIG. 1 illustrates the case of producing a press formed product having a hat-shaped cross-section (see a dashed line in FIG. 1). The press forming apparatus 1 includes a punch 2 as a lower die, and includes a die 3 and a pad 5 as an upper die. The pad 5 is supported by the die 3 or by a die holder or slider integrally operated with the die 3 via a pressure member 4. The pad 5 can be received in the die 3, and constitutes a part of the die 3 with being received in the die 3.
The bending forming of forming a metal sheet 6 into a press formed product by such press forming apparatus 1 is performed as follows. Before the forming, the metal sheet 6 is sandwiched between the punch 2 and the pad 5. That is, before staring the pushing-in of the metal sheet 6 into the die 3 by the punch 2, a portion of the metal sheet 6, which portion is formed into a top panel part of the press formed product, is restrained by the punch 2 and the pad 5. In this state, the die 3 is descended to a bottom dead point. In this manner, the top panel part of the press formed product is formed along a top surface (a front end surface) 2a of the punch 2. Vertical wall parts are formed along side surfaces 2b of the punch 2. Ridge line parts are formed between the top panel part and the vertical wall parts. The ridge line parts connecting the top panel part to the vertical wall parts are formed along punch shoulder parts 2c of the punch 2. Hereinafter, such bending forming is referred to as the pad bending forming.
In these years, it is required for automobiles to save the car body weight, so as to improve the fuel economy, which contributes to prevention of global warming. Further, the improvement of the safety at the time of collision accident is required. Because of these requirements, a high-strength steel sheet having a tensile strength of 590 MPa or more is used as the metal sheet 6, which is the starting material of the structural member. For example, a 980 MPa-class high-strength steel sheet is also used, and in some cases, a 1180 MPa-class high-strength steel sheet is used.
Additionally, the shapes of the structural members may become relatively complex. This is because of the design constraints, such as prevention of the interference between the structural members and the other members, bonding between the structural members and the other members, securing of a desired space, etc.
FIGS. 2A and 2B and FIGS. 3 to 8 are perspective views showing examples of press formed products having relatively complex shapes. Among these figures, FIGS. 2A and 2B show a first exemplary press formed product 7 formed from a high-strength steel sheet having a tensile strength of 590 MPa or more (an alloyed hot-dip galvanized steel sheet made of DP (Dual Phase) steel having a sheet thickness of 1.2 mm). FIG. 2A shows the entire first exemplary press formed product 7, and FIG. 2B shows a portion A in FIG. 2A in an enlarged manner. Additionally, FIGS. 3 to 8 show second to seventh exemplary press formed products 7 formed from a high-strength steel sheet having a tensile strength of 590 MPa or more (an alloyed hot-dip galvanized steel sheet, a non-plated steel sheet, etc. having a sheet thickness of about 1.2 mm), respectively.
Each of the cross-section shapes of the first to seventh exemplary press formed products 7 is a hat shape. That is, each of the press formed products 7 includes a top panel part 7a, vertical wall parts 7c extending from the left and right ends of the top panel part 7a, ridge line parts 7b between the top panel part 7a and the vertical wall parts 7c, and flange parts 7d extending from the lower ends of the respective vertical wall parts 7c. Further, the press formed product 7 includes shape-changing parts 9 in parts of the ridge line parts 7b. 
In the first exemplary press formed product 7 shown in FIGS. 2A and 2B, the shape-changing parts 9 are provided in both of the ridge line parts 7b on both ends of the top panel part 7a. In these shape-changing parts 9, the height of the ridge line parts 7b is changed at local portions located in almost middle of the lengthwise direction of the press formed product 7. In this case, a step height part 8 is formed in the top panel part 7a in a region connecting the shape-changing parts 9 to each other.
In the second exemplary press formed product 7 shown in FIG. 3, the shape-changing parts 9 are provided in both of the ridge line parts 7b on both ends of the top panel part 7a. In these shape-changing parts 9, the height of the ridge line parts 7b is changed in a wide area almost in the middle of the lengthwise direction of the press formed product 7. In this case, the height of the top panel part 7a gently changes corresponding to the positions of the shape-changing parts 9.
In the third exemplary press formed product 7 shown in FIG. 4, the shape-changing part 9 is provided in one of the ridge line parts 7b on both ends of the top panel part 7a. This shape-changing part 9 change the arc length in a cross section of the ridge line part 7b at a local portion almost in the middle of the lengthwise direction of the press formed product 7. In this case, the angle between the top panel part 7a and the vertical wall part 7c changes corresponding to the position of the shape-changing part 9.
In the fourth exemplary press formed product 7 shown in FIG. 5, the shape-changing parts 9 are provided in both of the ridge line parts 7b on both ends of the top panel part 7a. In these shape-changing parts 9, the ridge line parts 7b are twisted in a wide area almost in the middle of the lengthwise direction of the press formed product 7. In this case, the top panel part 7a and the vertical wall parts 7c are twisted corresponding to the positions of the shape-changing parts 9.
In the fifth exemplary press formed product 7 shown in FIG. 6, the shape-changing parts 9 are provided in both of the ridge line parts 7b on both ends of the top panel part 7a. In these shape-changing parts 9, the ridge line parts 7b are warped in the width direction of the press formed product 7 in a wide area almost in the middle of the lengthwise direction of the press formed product 7. In this case, the top panel part 7a and the vertical wall parts 7c are warped in the width direction of the press formed product 7 corresponding to the positions of the shape-changing parts 9.
In the sixth exemplary press formed product 7 shown in FIG. 7, the shape-changing part 9 is provided in one of the ridge line parts 7b on both ends of the top panel part 7a. In the shape-changing part 9, this ridge line part 7b is hollowed in the width direction of the press formed product 7 at a local portion almost in the middle of the lengthwise direction of the press formed product 7. In this case, the top panel part 7a and the vertical wall parts 7c are hollowed in the width direction of the press formed product 7 corresponding to the position of the shape-changing part 9.
In the seventh exemplary press formed product 7 shown in FIG. 8, the shape-changing part 9 is provided in one of the ridge line parts 7b on both ends of the top panel part 7a. In this shape-changing part 9, the radius of curvature of a cross section of the ridge line part is changed in an area from almost the middle to one end of the lengthwise direction of the press formed product 7. In this case, the width of the top panel part 7a and the height of the vertical wall part 7c are changed corresponding to the position of the shape-changing part 9.
When cold producing the first exemplary press formed product 7 shown in FIG. 2A from a high-strength steel sheet by the pad bending forming shown in FIG. 1, the following problems occur. As indicated by bold lines in FIG. 2B, in the press formed product 7, cracks occur in the region of the shape-changing part 9 of the ridge line part 7b. These cracks also occur in the region in the vicinity of the shape-changing part 9 (the step height part 8, the top panel part 7a, etc.). At the time of the pad bending forming, the ridge line part 7b is subjected to bending stress in the direction perpendicular to the direction along the ridge line part 7b. Further, the shape-changing part 9 in the ridge line part 7b is subjected to tensile stress in the direction along the ridge line part 7b. That is, the shape-changing part 9 is subjected to both of these bending stress and tensile stress. Therefore, tensile and bending deformation occurs in the region of the shape-changing part 9 and in the region in the vicinity thereof. It is considered that the cracks occur in the region of the shape-changing part 9 due to this tensile and bending deformation.
Such cracks may similarly occur even in the second to seventh exemplary press formed products press formed product 7 shown in FIGS. 3 to 8.
Aiming to suppress the occurrence of such cracks reduces the degree of freedom in designing the structural members. This is because the various dimensions of the shape-changing parts 9 in the ridge line parts 7b are restricted in accordance with the press formability of a high-strength steel sheet, which is a starting material.
The prior arts for preventing the occurrence of cracks in press formed products are disclosed in, for example, International Application Publication No. WO2014/042067 (Patent Literature 1) and Japanese Patent Application Publication No. 11-319963 (Patent Literature 2).